The present invention relates to a method of and an apparatus for tracking position error control in a closed-loop servo system, and more particularly to a method of and an apparatus for controlling a tracking position error so as to be of a desired value in a positioning servo system for a machine tool or the like.
Prior positioning servo systems for use with machine tools or the like control the position of an object, such as a workpiece table, with the difference between feed command pulses and position feedback pulses which is applied as an actual speed command. The track acutally followed by the object is delayed at all times with respect to the actually commanded position, the delay varying dependent on the commanded feed speed and the gain of the closed servo loop.
Ordinary digital servo systems comprise a differential counter for counting feed command pulses and position feedback pulses and issuing the difference as an output signal indicating a speed command, and a D/A converter for converting the output signal from the differential counter to an analog signal. The output signal of the differential counter is representative of the delay of the load shaft position from the commanded position, and depends on the commanded feed speed and the closed-loop gain.
When a workpiece is to be machined along a given path by controlling two or more axes in a machine tool, the contour to which the workpiece is machined is subjected to an error if the gains of the closed servo loops for the respective axes about which the workpiece is displaced are different from each other. To avoid this drawback, the controls for conventional machine tools have adjustable axis loop gains for achieving a desired degree of machining accuracy. However, when any contouring error is to remain in the range of submicrons, the desired machining accuracy could not be accomplished only by adjusting the loop gains for the respective axes. Particularly, it has been found in ultra-precision machine tools that the contouring error is produced by different delays caused with respect to the respective axes when the loop gains in the control servo system are different from axis to axis even if the same command speed is given to the axes.